This invention relates to an acoustic vibration generating element. In particular, this invention is suitable for a bone conduction device, such as a bone conduction speaker, for converting an acoustic electric signal into acoustic vibration to be transmitted to a part of a human body, such as a cranial bone or an arm, so that an acoustic sound is sensed by an auditory nerve.
Heretofore, as an electromechanical transducer for a bone conduction device, use is predominantly made of an electromagnetic type. The electromechanical transducer of the electromagnetic type utilizes a principle same as that of a dynamic speaker and converts a driving force generated by interaction between an electric current flowing through a coil and a magnet into mechanical vibration. The electromechanical transducer of the type is disclosed, for example, in Japanese Patents (JP-B) Nos. 2967777 (corresp. to U.S. Pat. No. 6,141,427) and 3358086 (corresp. to U.S. Pat. No. 6,668,065).
However, the electromechanical transducer of the electromagnetic type is disadvantageous in the following respects. The electromechanical transducer of the electromagnetic type generates an electromagnetic force and therefore requires the electric current. When the electric current flows through the coil, an energy loss inevitably occurs by a resistance of the coil. Thus, most of energy supplied from a power source is dissipated at the coil as Joule heat and only about 1% of the energy is used as acoustic energy. Further, in a low-frequency region, the electric current tends to be excessive because of low impedance so that a load upon the power source is increased. As a result, a sound output level is inevitably limited in the low-frequency region. Thus, in the low-frequency region, the sound output level tends to be insufficient.
On the other hand, proposal is made of a transducer for a bone conduction device, i.e., a bone conduction transducer which uses a piezoelectric element, although in a minority. The bone conduction transducer using the piezoelectric element comprises, as an acoustic vibration generating element, a piezoelectric unimorph element which is often used as a piezoelectric sound generator. The piezoelectric unimorph element comprises a metal plate and a piezoelectric plate adhered thereto. The bone conduction transducer of the type is disclosed, for example, in Japanese Patent Application Publications (JP-A) Nos. S59-140796 and S59-178895.
However, the bone conduction transducer using the piezoelectric element is disadvantageous in the following respects. Specifically, the bone conduction transducer using the piezoelectric element has a resonance frequency of 1 kHz or more if the bone conduction transducer has a practical size. Therefore, reproduction in the low-frequency region lower than the resonance frequency is insufficient. Further, since a mechanical quality factor Q of a vibrating system is high, generation of vibration is emphasized or attenuated at a specific frequency. In this event, sound reproduction can not naturally and normally be carried out.
As an example of the bone conduction device, there is also provided a bone conduction speaker not for a hearing-impaired person but for an unimpaired person. The bone conduction speaker of the type is required to prevent a reproduced sound from leaking to others except a user. However, with a known structure of the bone conduction speaker, vibration of a vibration source propagates to a structural member. As a result, the vibration of the structural member is propagated to the surroundings as the reproduced sound.
In bone conduction applications of the piezoelectric element, a resonance frequency of the piezoelectric element must be as low as possible if the low-frequency region is regarded as important. In order to lower the resonance frequency of the piezoelectric element, the following techniques A to C are proposed.
A. To increase a diameter or a length of the piezoelectric element, which determines a vibration mode.
B. To lower a flexural modulus K of the piezoelectric element.
C. To add a mass to an antinode of vibration.
However, if an object equipped with the piezoelectric element is a portable apparatus such as a mobile phone and, therefore, the size of the piezoelectric element is restricted, the technique A has limitations.
The technique B is achieved by reducing the thickness of a piezoelectric ceramics sheet in case of a piezoelectric unimorph element and by reducing the thickness of a metal plate (shim plate) interposed between two piezoelectric ceramics sheets in case of a piezoelectric bimorph element. However, in this technique, the mechanical strength of the piezoelectric element is lowered. In addition, the weight of the piezoelectric element itself is decreased so that the resonance frequency is increased. Therefore, no substantive effect is obtained. Alternatively, by selecting an organic material having a small elastic modulus as the shim plate, the flexural modulus K can be lowered to some extent. However, the organic material generally has a small specific gravity so that the weight of a whole of the piezoelectric element is decreased. Therefore, the resonance frequency tends to be increased.
The technique C of adding the mass is disadvantageous in that the mechanical strength tends to be weakened against shocking vibration.
In a piezoelectric transducer such as the above-mentioned bone conduction transducer using the piezoelectric element, mechanical vibration is driven by piezoelectric distortion which is caused by an electric voltage. Therefore, the piezoelectric transducer is not accompanied with dissipation of Joule heat by the coil in the above-mentioned electromechanical transducer of the electromagnetic type. Therefore, it is possible to achieve energy saving. In addition, since metal components such as a magnet and a yoke are not required, a light weight and a thin profile can be achieved. Thus, the piezoelectric transducer has many advantages. In order to fully enjoy those advantages, the piezoelectric transducer is required to overcome the disadvantages such as a high resonance frequency and a high mechanical quality factor Q.
On the other hand, prevention of sound leakage to the surroundings is an unavoidable issue in order to bring the bone conduction speaker into practical use, whether electromagnetic or piezoelectric. In order to further exhibit the characteristics of the piezoelectric transducer, an input driving voltage is preferably suppressed as low as possible. In this event, energy loss of a driving circuit combined with the piezoelectric transducer is advantageously suppressed.
Further, the bone conduction speaker is generally attached to a human head when it is used. Therefore, it is desired for a user that the bone conduction speaker is light in weight and is easily wearable.